GE 469 Instruction Manual

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Contents

S5 Thermal Model469Motor Management RelaySetpointshttp://www.GEindustrial.com/multilin5–46 GE Multilincreating an equivalent motor heating current rather than simply using averagecurrent (Iper_unit). This equivalent current is calculated as shown below.(EQ 5.3)where: I eq = equivalent motor heating currentIper_unit = per unit current based on FLAI2 = negative sequence current, I1 = positive sequence currentk = constantThe figure below shows recommended motor derating as a function of voltageunbalance recommended by NEMA (the National Electrical ManufacturersAssociation). Assuming a typical induction motor with an inrush of 6 x FLA and anegative sequence impedance of 0.167, voltage unbalances of 1, 2, 3, 4, and 5%equal current unbalances of 6, 12, 18, 24, and 30% respectively. Based on thisassumption, the GE Multilin curve illustrates the motor derating for different valuesof k entered for the UNBALANCE BIAS K FACTOR setpoint. Note that the curve createdwhen k = 8 is almost identical to the NEMA derating curve.FIGURE 5–14: Medium Motor Derating Factor due to Unbalanced VoltageIf a k value of 0 is entered, the unbalance biasing is defeated and the overload curvewill time out against the measured per unit motor current. k may be calculated as:(EQ 5.4)where ILR is the per-unit locked rotor current.f) Motor CoolingThe thermal capacity used value decreases exponentially when the motor current isless than the OVERLOAD PICKUP setpoint. This reduction simulates motor cooling. Themotor cooling time constants should be entered for both stopped and running cases.Since cooling is exponential, the time constants are one-fifth of the total time from100% thermal capacity used to 0%. A stopped motor normally cools significantlyslower than a running motor. Motor cooling is calculated as:(EQ 5.5)(EQ 5.6)where: TC used = thermal capacity usedTC used_start = TC used value caused by overload conditionTC used_end = TC used value dictated by the hot/cold safe stall ratio whenthe motor is running (= 0 when the motor is stopped)t = time in minutesτ = Cool Time Constant (running or stopped)I eq = equivalent motor heating currentol_pkp = overload pickup setpoint as a multiple of FLAhot / cold = hot/cold safe stall ratioI eq I2per_unit 1 k I2I1-----   2⋅+  ⋅=NEMA0.700.750.800.850.900.951.001.050 1 2 3 4 5PERCENT VOLTAGE UNBALANCEDERATING FACTOR808728A1.CDRk=2k=4k=6k=8k=100.700.750.800.850.900.951.001.050 1 2 3 4 5PERCENT VOLTAGE UNBALANCEDERATING FACTORGE MULTILINk 175I LR2-----------= (typical estimate); k 230I LR2-----------= (conservative estimate)TC used TC used_start TC used_end–( ) e t τ⁄–( ) TC used_end+=TC used_endI eqol_pkp--------------------   1 hotcold------------–   100%×=